The present invention relates to a level sensor which is developed so as to determine the undershooting or overshooting of a predetermined level of liquid in a container, the sensor having a resistance element which is traversed by electric current and changes its electrical resistance upon an increase in the temperature, and having a measuring unit for measuring the electric resistance of the electrical conductor. The invention also relates to a method of determining the undershooting or overshooting of an intended level of liquid in a container, in which method a resistance element which is arranged at the intended level of filling and changes its electrical resistance with an increase in temperature is heated by an electric current whereupon its electrical resistance is measured.
Such limit-level sensors are frequently used in motor vehicles, for instance in a transmission, an internal combustion engine or a fuel tank and are known from actual practice. As resistance element, the limit-level sensor has a resistance wire which extends down into the region of the bottom of a container to be measured. For the measurement, the resistance wire is traversed by an electric current. In this way, the resistance wire is heated, thereby changing its electrical resistance. Since the heat produced thereby in the region of the resistance wire immersed in the liquid is substantially more strongly led away from the liquid than the region above the liquid, the level of the liquid can be calculated from the resistance ascertained and compared with a minimum level.
This method has the disadvantage that the resistance of the resistance wire which is determined is influenced by the temperature of the liquid. The determination of the level is thus relatively inaccurate. Therefore, a measurement of the minimum level of motor oil in an internal combustion engine of a motor vehicle having this level sensor is possible only at great expense.
For the dependable determination of the level of a liquid having a particularly high temperature, floats provided with a magnet which float on the liquid are known. When an intended level is undershot or overshot, the magnet switches a reed contact which is arranged at the height corresponding to the level. Such a limit-level switch, however, is particularly expensive. Furthermore, this level switch has movable parts which can jam.
The object of the present invention is to develop a limit-level sensor of the aforementioned type in such a manner that it has no movable parts and that the overshooting or undershooting of a level of a liquid can be reliably determined with it even at high temperatures. Furthermore, a method is to be provided for determining the undershooting or overshooting of an intended height of filling of liquid in a container.
The first-mentioned problem is solved in accordance with the invention in the manner that the resistance element is arranged exclusively at the intended height of filling and has an electrical resistance which changes suddenly above a transition temperature which lies above the maximum temperature of the liquid.
The resistance element is first of all heated electrically like the resistance wire of the known level-limit sensor. If the heat produced thereby is removed very rapidly, the resistance element is sufficiently covered by liquid. Since air removes the heat very poorly from the resistance element, the resistance element will heat up to a temperature above the transition temperature (for instance a PTC element) in case of too low a level of filling and initially retain this temperature. In this way, a resistance value which has been determined can be ascribed unequivocally to a temperature range of the resistance element which lies above or below the transition temperature and an overshooting or undershooting of the level of the liquid can thus be determined. Since the temperature of the liquid lies below the transition temperature of the resistance element, its influence on the result of the measurement is negligible.
In accordance with an advantageous further development of the invention, the limit-level sensor is suitable for use at particularly high temperatures, if the resistance element is a PTC element. Such a PTC element has a particularly high transition temperature of about 250xc2x0 C. Up to the transition temperature, the electrical resistance of the PTC element decreases slightly with an increase in temperature. Above the transition temperature, the electrical resistance increases abruptly.
The electrical resistance of the resistance element can be determined in a particularly simple manner, in accordance with an advantageous further development of the invention, if the resistance element is connected in series with a series resistor. In this way, the voltage is measured on the resistance element. With the total voltage on the series resistor and the resistance element known, the voltage measured is dependent directly on the electrical resistance of the resistor element.
The mounting of the resistance element is particularly simple in accordance with another advantageous development of the invention if the limit-level sensor has a housing which can be assembled from two housing parts and if connecting wires leading to the resistance element are clamped between the two housing parts.
A discharge of the heat produced by the heating of the resistance element is reliably prevented, in accordance with another advantageous development of the invention, if the resistance element is soldered or welded to the connecting wires, and if the housing has exclusively points of contact with the connecting wires. Furthermore, this development has the advantage that liquid or can flow freely around the resistance element.
In accordance with another advantageous feature of the invention, the housing can be produced particularly inexpensively if one of the housing parts has a plurality of thin-wall transverse ribs which have grooves to receive the connecting wires and if the second housing parts has projections which lie on the connecting wires. Furthermore, by this development, the removal of heat via the connecting wires and the housing is kept particularly slight.
Particularly at low temperatures and with too low a level, a drop of liquid adhering between the resistance element and the housing can lead the heat away from the resistance element and result in an erroneous measurement. Such an erroneous measurement can be dependably avoided in accordance with another advantageous feature of the invention if there is a distance between the resistance element and the regions of the housing which are adjacent it.
The resistance element which is held exclusively by the connecting wires requires particularly strong connecting wires for an oscillation-resistant attachment. A discharge of the heat of the resistance element over the connecting wires is kept particularly slight in accordance with another advantageous development of the invention if the connecting wires are made, for instance, of constantan.
A minimum level and a maximum level can be easily determined by another advantageous development of the invention if the limit-level sensor has two resistance elements arranged at different heights.
The second problem, namely the creation of a method for the determining an undershooting or overshooting of an intended level of liquid in a container is solved, in accordance with the invention, in the manner that upon the heating, the resistance element is fed as much electrical energy as is necessary in order to exceed a transition temperature of the resistance element present in air which abruptly changes the electrical resistance.
By this method, a value which can be unambiguously associated with the region above or below the transition temperature, is obtained upon the measurement of the electrical resistance of the resistance element. If the resistance element extends into the liquid, then the heat produced by the electric current is led away by the liquid and the resistance element has a temperature, and thus a corresponding value of resistance, below the transition temperature. In the event that the resistance element does not extend into the liquid, the heat is not led away, so that the temperature of the resistance element remains above the transition temperature. A further advantage of this method is that liquid which adheres to the resistance element is first of all also heated. This leads, for instance in the event of cold oil, to a reduction in its viscosity and thus to a dripping of oil from the resistance element. In this way, erroneous measurements are reliably prevented.
The method of measurement requires particularly little time in accordance with an advantageous further development of the invention if the resistance element is heated with a electric current via a series resistance and the voltage then measured on the resistance element. The voltage measured on the resistance element is dependent on the electrical resistance of the resistance element.
A limit-level sensor used for the determination of a minimum level of motor oil in an internal combustion engine of a motor vehicle is particularly reliable if the heating of the resistance element is commenced about five to ten minutes after the disconnecting of the internal combustion engine.